The art of lithographic printing is based upon the immiscibility of oil and water, wherein an oily material or ink is preferentially retained by an imaged area and the water or fountain solution is preferentially retained by the nonimaged areas. When a suitably prepared surface is moistened with water, and ink is then applied, the background or nonimaged areas retain the water and repel the ink while the imaged areas accept the ink and repel the water. The ink is eventually transferred to the surface of a suitable substrate, such as cloth, paper or metal, thereby reproducing the image.
Very common lithographic printing plates include a metal or polymer support having thereon an imaging layer sensitive to visible or UV light. Both positive- and negative-working printing plates can be prepared in this fashion. Upon exposure, and perhaps post-exposure heating, imaged or nonimaged areas are removed using wet processing chemistries.
Thermally sensitive printing plates are less common but becoming more prominent. Examples of such plates are described in U.S. Pat. No. 5,372,915 (Haley et al). They include an imaging layer comprising a mixture of dissolvable polymer s and an infrared radiation absorbing compound. While these plates can be imaged using lasers and digital information, they require wet processing using alkaline developer solutions after imaging.
Thus, conventional preparation of lithographic printing plates generally involves multiple processing steps such as exposure to irradiation and subsequent chemical processing. "Direct write" eliminates the use of patterned light image and the process used to generate such image. "Direct write" using an infrared laser is a thermally driven process and is more desirable because the imaging laser heats only small regions at a time. Moreover, computer control allows for high resolution images to be generated at high speed such the images can be produced directly on a printing plate pixel by pixel. The chemical processing steps are also avoided. Thus, thermal "direct write" processless lithographic printing plates and methods of images are in demand in the industry.
It has been recognized that a "direct write" lithographic printing plate could be created containing an IR absorbing layer. For example, Canadian 1,050,805 (Eames) discloses a dry planographic printing plate comprising an ink receptive substrate, an overlying silicone rubber layer, and an interposed layer comprised of laser energy absorbing particles (such as carbon particles) in a self-oxidizing binder (such as nitrocellulose). The plate was developed by applying naphtha solvent to remove debris from the exposed image areas. Similar plates are described in Research Disclosure 19201, 1980 as having vacuum-evaporated metal layers to absorb laser radiation in order to facilitate the removal of a silicone rubber overcoated layer. These plates were developed by wetting with hexane and rubbing. Other ablation imaging processes are described for example in U.S. Pat. No. 5,385,092 (Lewis et al), U.S. Pat. No. 5,339,737 (Lewis et al), U.S. Pat. No. 5,353,705 (Lewis et al), US Reissue 35,512 (Nowak et al) and U.S. Pat. No. 5,378,580 (Leenders).
While the noted printing plates used for digital, processless printing have a number of advantages over the more conventional photosensitive printing plates, there are a number of disadvantages with their use. The process of ablation creates debris and vaporized materials that must be collected. The laser power required for ablation can be considerably high, and the components of such printing plates may be expensive, difficult to coat, or unacceptable in resulting printing quality. Such plates generally require at least two coated layers on a support.
A variety of materials and methods have been used to prepare thermal direct write lithographic printing plates. Thermal direct write plates that require an aqueous processing step are known. For example, U.S. Pat. No. 5,512,418 (Ma) describes the use of cationic polymers containing pendant ammonium groups for thermally-induced imaging. However, such plates require aqueous processing steps after imaging.
Similarly, U.S. Pat. No. 4,693,958 (Schwartz et al) discloses a method of preparing litho printing plates which also require aqueous processing by using polyamic acids and vinyl polymers containing pendant quaternary ammonium groups. Nonthermal wet processing printing plates are also reported in U.S. Pat. No. 4,405,705 (Etoh et al). Resin composition comprising basic polymers and organic carboxylic acids are exposed to ultraviolet lights and developed with water to produce negative-working plates.
U.S. Pat. No. 4,081,572 (Pacansky) describes the preparation of lithographic printing masters employing hydrophilic polyamic acids that can be selectively converted to hydrophobic polyimides imagewise by heat. However, the laser exposure was applied through a transparency mask, that is an image-bearing transparency and hence it is not a "direct write" printing plate.
Processless plates have also been prepared by changing the surface tension of resin compositions as described in U.S. Pat. No. 4,634,659 (Esumi et al). Photooxidation-sensitive resins such as polystyrene and polyethylene were exposed to ultraviolet lights and the imaged area became hydrophilic to repel ink due to the oxidation and the roughness of the surface. However, this is not a thermal process. Altering surface tension has also been applied to U.S. Pat. No. 4,034,183 (Uhlig). This patent discloses the method to produce thermal direct write processless plates using a high power laser. However, printing plates prepared using differentiation of surface tension suffer from poor physical properties and limited run lengths.
U.S. Pat. No. 3,650,743 (Hallman et al) and U.S. Pat. No. 4,115,127 (Ikeda et al) describe methods of preparing processless litho printing plates using inorganic materials. However, both imaging members are multiple-layer structures and some of them use toxic materials such as arsenic and others require vacuum deposition of mixed inorganic coating materials. Moreover, both are not thermally-imageable plates.
Thermal or laser mass transfer is another method of preparing processless litho printing plates. U.S. Pat. No. 5,460,918 (Ali et al) discloses a process of thermally transferring a hydrophobic image from a donor sheet to a microporous hydrophilic crosslinked silicated surface of the receiver sheet. In another example, U.S. Pat. No. 3,964,389 (Peterson) describes a process of laser transferring an image from donor to receiver but requires high temperature postheat. Both processes require donor and receiver sheets and have practical disadvantages of maintaining extremely clean surfaces during transfer.
U.S. Pat. No. 5,569,573 (Takahashi et al) describes a new method for production of thermal direct write processless litho printing plates. The coating comprises a hydrophilic three-dimensional cross-linked binder and a microcapsuled hydrophobic material. Upon heating, the microcapsule ruptures and forms a hydrophilic image.
Thermally switchable polymers have been described for use as imaging materials in printing plates. By "switchable" is meant that the polymer is rendered either more hydrophobic or hydrophilic upon exposure to heat. EP-A 0 652 483 (Ellis et al) describes a process of preparing thermal direct write processless plates using polymers containing acid- or heat-labile pendant hydrophobic groups which becomes hydrophilic upon heating. However, polymers of this kind suffer from short shelf life and are difficult to manufacture.
Up until the present, heat-sensitive polymers used in printing plates are linear polymers. There is a need to provide heat-sensitive materials that are more durable and heat-sensitive in the imaging and printing operations.
Compared with linear polymers, dendritic polymers (dendrimers) provide some unique advantages (Frechet et al, Science, 1995, 269, 1080). First, the intrinsic viscosity of dendrimer is lower compared with linear analog with the same molecular weight. Second, the level of interaction between solvent and polymer is decreased and polymer becomes much more compact. Third, if the functional groups are located at the termini of dendrimer, the functional group becomes more accessible and occupies much higher surface area.
Since the regularly branched dendrimers were prepared only through lengthy multi-step syntheses, their availability is limited to a small group of functional monomers and industrial production of dendrimers is therefore limited.
Compared to a dendrimer, a hyperbranched polymer is less regular. However. it might approximate at least some of the desirable properties of dendrimers (Frechet et al. J. Macromol. Sci., Pure Appl. Chem. 1996, A33, 1399). More importantly, hyperbranched polymers are more conducive to industrial applications. Hyperbranched polymers made by condensation reactions have been suggested (Kim, et al., J. Am. Chem. Soc. 1990, 112, 4592, and Hawker, et al. ibid, 1991, 113, 4583). Frechet et al discovered that a large numbers of vinyl monomer based hyperbranched polymers can be obtained by means of living chain polymerization of branching vinyl monomer (U.S. Pat. No. 5,587,441 and U.S. Pat. No. 5,587,446 both of Frechet et al). Since their discovery, various vinyl hyperbranched polymers have been prepared by living cationic polymerization (U.S. Pat. No. 5,587,441), atom transfer radical polymerization (U.S. Pat. No. 5,763,548 of Wang et al), group transfer polymerization [Muller et al, Polymer Preprint, 1997, 38(1), 4981], and stable radical polymerization (Hawker et al, J. Am. Chem. Soc. 1991, 11, 4583). The resultant vinyl hyperbranched polymer from living chain polymerization is a totally different class of materials from the dendrimer and its derivatives in terms of both chemical composition and macromolecular architecture.
Vinyl hyperbranched polymers with different structures, such as random copolymer (Gaynor, et al. Macromolecules, 1996, 29, 1079), grafted hyperbranched copolymer (U.S. Ser. No. 09/105,767, Kodak Docket No. 77710), and block hyperbranched copolymer (U.S. Ser. No. 09/105,765, Kodak Docket No. 77708), have been made by atom transfer radical polymerization process.
The graphic arts industry is seeking alternative means for providing a direct write, negative- or positive-working lithographic printing plate with high sensitivity, high imaging speed, long shelf life and press life, that can be imaged without ablation and the accompanying problems noted above. The heat-sensitive polymers used for this purpose until this time have not fully met all of the needs of the industry.